TITK PKOTRACTOU PEDIS MUSCLE. 
The idea tliat the small muscle passing inwards from the " spot " — which in position 
is a good deal anterior to the functional adductor — might be the reduced remains of the 
larval anterior adductor, is at once negatived by the observation that the fibres do not pass 
through the body from side to side, as adductor fibres must ; but that we have to do with 
two distinct muscles, one on each side of the body. This, then, suggests the muscles for 
moving the " foot," and although there is no foot in the adult oyster, there can be no doubt 
that Ryder and Jackson were right in referring to this as a " pedal " muscle. 
The muscles which may be present in other Lamellibranchs in relation to the foot 
are a protractor and an anterior and a posterior retractor on each side. The posterior 
retractor is out of the question. The protractor and the anterior retractor are usuall}- 
inserted into the shell a little posterior to the anterior adductor muscle, and the protractor 
is the more ventrally placed of the two. The hinge line of the oyster is anterior, and if we 
may judge from Horst's figure of the larval oyster at the stage when the foot, as a ciliated 
ventral projection between the mouth and the anus, is in its most fully developed state, a 
protractor muscle would run posteriorly from in front of the mouth or oesophagus ; while a 
retractor would, from what we know of it in other molluscs, probably be placed more 
dorsally, so as to cross the stomach. Our muscle runs antero-posteriorly, and is towards 
the ventral part of the visceral mass, close to the insertion of the labial palps. Such 
indications as we have, then, point to the " spot " being the place of insertion of the modified 
remains of the protractor pedis muscle. 
The foot is however lost in the oyster at a very early period — while the larva is still 
free-swimming ; and if the protractor muscle is present in the adult in this relatively large 
condition, it must be'because it has become adapted to a new purpose. We have traced 
the muscle fibres in the American oyster by means of serial sections from the insertion into 
the shell (PI. II., Figs. 9 and 10) at the "spot," posteriorly along the top or attached edge of 
the outer labial palp, to the anterior end of the branchial lamellze. The fibres first run 
inwards, and then obliquely backwards (Fig. 1 1 ), and come into close relation with a large 
blood sinus {see Pi. II., Figs. 11, 12, 13, /ac), which lies in the mantle just over the attachment 
of the outer labial palp. The muscle then works its way round the sinus, so as to appear 
upon its ventral, and finally its inner face (Fig. 12). It thus comes to lie internal to some of 
the renal tubules, and close to the outermost ca;ca of the liver. Our muscle now cro.sses at 
right angles a bundle of muscle fibres running from the mantle downwards ventrally into 
the palp, and containing a large nerve, as is usual with the muscle bundles of the 
mantle. Finally, having reached the level of the posterior end of the palp, our muscle 
turns ventrally, and begins to spread out (Fig. 13) in the sub-epithelial tract of mesodermal 
tissue which lies between the top of the mantle lobe and the attachment of the gill lamella;, 
and which is m.orphologically a part of the stem of the ctenidium. Most of the fibres end 
in the connective tissue, immediately under the epidermis, between the descending (internal) 
and the ascending (external) lamella; of the outer gill. The whole course of the muscle is 
about 6 mm. long in a moderately large oyster; it is about 1-5 mm. in diameter 
where inserted into the shell, and less than half that size when it reaches the hranchi.e. 
